1. Field of the Invention
This invention relates to a process for making water soluble vinyl caprolactam polymers, and, more particularly, to a homopolymer of vinyl caprolactam, and copolymers with dimethylaminopropyl methacrylamide, optionally with one or more polymerizable monomers, by suspension polymerization in an aqueous/alcoholic solvent medium, without requiring a protective colloid, which polymers are particularly suitable for use as a coatings for color-inkjet receptive films.
2. Description of the Prior Art
J. Kroker et al, in U.S. Pat. No. 5,739,195, described a process for preparing an aqueous solution of 10-60% polyvinyl caprolactam (PVCL) homopolymer at a temperature above its cloud point in the presence of 0.1-20% by weight based upon vinyl caprolactam (VCL) monomer of a water-soluble synthetic polymeric protective colloid. Aqueous polyvinyl caprolactam homopolymer made with a protective colloid present in the starting materials was homogeneous, whereas protective colloid free systems were inhomogeneous, which were not readily dilutable with water except stirring for a prolonged period of time. Addition of an emulsifier to the starting material also formed an appreciable portion of PVCL polymer remained attached to the stirrer element.
The advent of color inkjet printing has been instrumental in fueling the print-on-demand revolution and has also created a number of challenges. Often, the surface of the desired media does not possess the necessary properties for accepting the ink-jet ink. This results in long dry times and/or a poor ink-jet image. It has long been recognized that a surface treatment or media coating plays a critical role in the final print quality. Numerous media coatings are known in the art. They may contain any number of components and often consist of more than one layer. These ink-receptive coatings generally contain at least one hydrophilic polymer; often poly(vinylpyrrolidone) (PVP). PVP brings many benefits to properly formulated media coatings including rapid ink dry time, excellent print quality, highly resolved circular dots, and high, uniform optical density. Furthermore, copolymers of vinylpyrrolidone (VP) along with other suitable comonomers, such as dimethylaminoethyl methacrylate, acrylic acid, or vinyl acetate, have been used separately or in conjunction with PVP, to further optimize performance. Unfortunately, ink-jet images printed on VP-based formulations are prone to fading with exposure to light. Increasing demands for achievable ink-jet images has created a need for polymers with all the positive attributes of vinyl pyrrolidone and with improved light stability.
A process of making a homopolymer of vinyl caprolactam (VCL), and a copolymer with dimethylaminopropyl methacrylamide (DMAPMA), optionally including with one or more polymerizable monomers (X), in the composition range, by weight, VCL/DMAPMA/X, 99 to 30%, 1-70% and 0-30%, respectively, by suspension polymerization in an aqueous/alcoholic solvent medium, without requiring a protective colloid, which comprises charging vinyl caprolactam in an aqueous/alcoholic solvent mixture, wherein the weight ratio of alcohol to total monomers is ⅙ to 3, and if present, forming a solution of DMAPMA and optional monomers in water, feeding the solution into the VCL charge at a predetermined rate, periodically adding initiator to the reaction mixture, and polymerizing at about 40xc2x0 to 130xc2x0 C., and, if desired, neutralizing the product, to form a polymer product at about 5% to 40% solids.
In accordance with the invention, the following are the General Reaction Conditionsxe2x80x9d for carrying out the suspension polymerization process herein without requiring a protective colloid.
Composition VCl/DMAPMA/X
VCl: 99 to 30 wt. %
DMAPMA: 1-70 wt. %
X: Any polymerizable monomer, 0-30 wt. %
Amount of Alcohol Solvent
Wt. ratio of alcohol/total monomers=⅙ to 3
Prefer: 0.8 to 1.2; methanol or ethanol
Feeding Time: DMAPMA:
1-6 hours; preferably 3-5 hours Feeding rate can be varied in order to get most homogeneous structure.
Polymerization Temperature:
40 to 130xc2x0 C.; prefer 50 to 80xc2x0 C.
Solids:
5 to 40%; prefer 10 to 20%
Neutralizer:
Organic acid: glycolic acid, citric acid, tataric acid, acetic acid
Inorganic acid: hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid
Initiator:
Any free radical initiator
Prefer: t-butylperoxypivalate, t-amylperoxypivalate, didecanoyl peroxide, Vazo(copyright) 52, 64, 67
Amount of Initiators:
Wt. ratio of initiator/total monomer=0.1% to 10%
Prefer: 0.2% to 5%
Most prefer: 0.3% to 1.0%